Ultrasonic acoustic wave type touch panels are in widespread use. Examples of their applications include operating screens of personal computers, ticket vending machines at train stations, copiers installed in convenience stores, and ATM's at financial institutions. These acoustic wave type touch panels utilize transducers (converters), including piezoelectric elements provided on a substrate formed of glass or the like. These transducers function both as generating means for acoustic waves and as sensors for detecting acoustic waves which are scattered by a finger or the like that contacts the touch panel.
The basic principle of acoustic wave touch panels is as follows. Ultrasonic vibrations, which are generated by transmitting side transducers as bulk waves, are converted to surface acoustic waves by mode converting elements, and are caused to propagate along the surface of the substrate. The transducers and reflective arrays are generally provided such that the surface acoustic waves propagate in the X axis direction and the Y axis direction of the substrate such that they intersect with each other. When the surface acoustic waves are blocked by a finger or the like along the paths thereof, the paths are blocked, and the surface acoustic waves are scattered. When the surface acoustic waves, of which the intensity has been reduced due to the scattering, enters receiving side transducers, they are detected as electric signals. The detected electric signals are referenced against clock signals of a controller separate from the touch panel, and the position at which the surface acoustic waves are blocked is determined.
However, when the bulk waves are converted to surface acoustic waves by the mode converting elements, not all of the bulk waves are converted into surface acoustic waves that propagate in predetermined directions. In addition, surface acoustic waves which are reflected by the reflective arrays are not necessarily reflected in the predetermined directions. These surface acoustic waves that do not propagate in the predetermined directions become so-called “spurious waves”. If these spurious waves propagate while being reflected along the substrate and reach the receiving side transducers, they cause the receiving side transducers to vibrate and generate voltages. These voltages are received as noise, and throw off proper judgment by the controller.
Various measures have been proposed to alleviate this problem, which is caused by the spurious waves. For example, providing vibration insulating or vibration absorbing materials on a substrate to absorb the generated spurious waves has been disclosed in European Patent Publication No. 0190734 (refer to page 11 and FIG. 2). Ultrasonic waves generated by transducers are emitted toward reflective arrays, but these ultrasonic waves include components that propagate in directions opposite the reflective arrays. The vibration absorbing materials are provided to attenuate the energy of these components. Spurious surface acoustic waves that propagate in directions opposite the reflective arrays are attenuated by the vibration absorbing materials, and prevent scattered reflection into a touch region.
As another example, U.S. Patent Application Publication No. 20030146673 discloses a touch panel having elastic material for absorbing surface acoustic waves provided on a substrate of the touch panel (refer to page 3, FIG. 2, and FIG. 3). This touch panel does not have any reflective arrays, and instead is equipped with transmitting side piezoelectric elements and receiving side piezoelectric elements that face each other across the substrate. The elastic material is provided closer to the edge of the substrate than the transmitting side piezoelectric elements and closer to the edge of the substrate than the receiving side piezoelectric elements. Thereby, acoustic waves that propagate from the transmitting side piezoelectric elements toward the edge of the substrate opposite the receiving side piezoelectric elements are attenuated. In addition, acoustic waves that have reached the receiving side piezoelectric elements from the transmitting side piezoelectric elements are attenuated after passing through the receiving side piezoelectric elements. Therefore, the acoustic waves are prevented from being reflected by the edge of the substrate to reenter the receiving side piezoelectric elements. Interference between predetermined functional acoustic waves which are to be utilized and spurious waves can be prevented by this construction.
As still another example, U.S. Pat. No. 7,204,148 discloses a touch panel provided with a diffraction grating comprising a great number of inclined lines as a spurious wave scattering means outside reflective array regions, that is, closer to the edges of a substrate than reflective arrays (refer to page 5, FIG. 1, and FIG. 10). When spurious acoustic waves reach the inclined lines of the diffraction grating, they are diffused and attenuated such that they do not reach receiving side converters. The diffraction grating attenuates spurious waves which have propagated beyond the reflective arrays toward the edges of the substrate, and spurious waves that propagate from transmitting side converters to the receiving side converters outside the reflective arrays, that is, in the vicinity of the edges of the substrate.
The aforementioned touch panels are designed only to remove spurious waves that propagate from the reflective arrays thereof toward surfaces of the substrate where objects contact the substrate, that is, touch regions, by indirectly attenuating the spurious waves. In other words, the aforementioned touch panels cannot prevent spurious waves from propagating toward the touch regions from the reflective arrays.
Generally, acoustic waves reflected by reflective arrays toward touch regions are reflected by inclined lines of the reflective arrays at 90° angles with respect to the incident direction of the acoustic waves, that is, the arrangement direction of the inclined lines. However, not all of the acoustic waves are reflected at exactly 90° angles. In actuality, there are acoustic wave components which are reflected toward directions slightly shifted from 90°. If these components propagate into the touch regions as spurious waves, they become signal noise which is different from signals obtained from regular acoustic waves. This signal noise may cause so-called “coordinate skipping” phenomena, in which controllers cannot accurately recognize contact positions, where pointers such as fingers and pens contact the touch regions.
In order to prevent coordinate skipping phenomena, it is desirable to remove spurious wave components of acoustic waves which are reflected by reflective arrays toward touch regions within the reflective arrays when they are generated, such that they do not propagate toward the touch regions. The aforementioned known touch panels are capable of attenuating spurious waves that propagate from the reflective arrays in directions opposite the touch regions, and spurious waves which have passed through the touch regions, in the vicinity of the edges of the substrates. However, they are not capable of preventing spurious waves from being propagated from the reflective array regions.